Asphalt is a common material utilized for the preparation of paving and roofing materials. While the material is suitable in many respects, it inherently is deficient in some physical properties which it would be highly desirable to improve. Efforts have been made in this direction by addition of certain conjugated diene rubbers, ethylene containing plastics like EVA and polyethylene, neoprene, resins, fillers and other materials for the modification of one or more of the physical properties of the asphalt binder. Each of these added materials modifies the asphalt in one respect or another but certain deficiencies can be noted in all modifiers proposed. For example, some of them have excellent weather resistance, sealing and bonding properties but are often deficient with respect to warm tack, modulus, hardness and other physical properties; and some of them improve only the high temperature performance of asphalt, some only improve the low temperature performance of asphalt, while some lack thermal stability or mixing stability with asphalt.
Since the late 1960s, diene polymer rubbers such as styrenebutadiene rubber and styrene-rubber block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers have been used to dramatically improve the thermal and mechanical properties of asphalts. Practical application of the rubber addition approach requires that the blended product retain improved properties and homogeneity during transportation, storage and processing. Long term performance of elastomermodified asphalts also depends on the ability of the blend to maintain thermal and chemical stability.
To be suitable for paving materials, the asphalt polymer mixtures should meet the following requirements:
(a) The polymer must be mixable in asphalt and stay mixed during subsequent processing.
(b) The mixture must have the right rheological (flow) properties to prevent rutting which is the permanent deformation of a road caused by repetitive traffic loads. Viscosity is important but elasticity is the most important aspect since the material must be able to recover rather than just resist deformation. This characteristic is most important in warm climates.
(c) The mixture must have good low temperature properties, i.e. resistance to cracking. As a road cools, stresses develop because it cannot shrink uniformly and eventually this will cause cracking. Traffic-caused stresses also contribute. The polymer will lower the temperature at which cracking will occur. This characteristic is more important in cold climates.
These polymers are necessary to obtain improved bituminous compositions, specifically for paving, and specifically for hot mix asphalt concrete (HMAC) and joint sealants which are used to fill in cracks in existing pavements. The polymer is added to the bitumens for hot mix asphalt concrete to improve its resistance to deformation caused by traffic loading especially at warm times of the year (rutting resistance), as well as improved resistance to cracking caused by both traffic and thermally induced loads, especially at cold times of the year. It is also necessary that the polymer be easy to mix with the bitumen and that the two do not demix during storage or processing. The polymers must be oxidatively stable during processing and during long term aging on the road.
Temperature susceptibility of a polymer modified asphalt is a major consideration. Ideally, one would want a binder (asphalt and polymer) which would be "hard" and elastic at elevated temperatures to resist permanent deformation and "soft" and strong at low temperatures to resist cracking. In other words, the ideal system should not only exhibit enhanced resistance to deformation but also should resist cracking.
To be suitable for synthetic roofing materials, the asphalt polymer mixtures should meet the following requirements:
(a) sufficient resistance to flow at high temperatures, PA1 (b) sufficient flexibility at low temperatures, PA1 (c) workability according to the conventional methods used in the roofing technique, PA1 (d) adequate hot storage stability, PA1 (e) adequate hardness to prevent deformation during walking on the roof, and PA1 (f) if it is to be used as an adhesive, sufficient adhesion.
For roll roofing applications, it is preferred that the softening point (the temperature at which the material will tend to flow) be above about 250.degree. F., the cold bend temperature, which is not as critical a parameter as the others in this application, (the temperature at which the material will crack during application and service) should be below about -5.degree. C. and that the asphalt and block copolymer components should be able to be mixed at a temperature no higher than about 200.degree. C. to keep the asphalt heating costs down and to prevent softening of the polyester reinforcement commonly used in these membranes.
For roll roofing membranes, the bituminous composition is used to saturate and coat a reinforcing mat. The bitumen is there to make the membrane waterproof. The mat is used to aid in mechanical properties (gives the membrane strength, etc.). Polymer is added to the asphalt to improve the weatherability and mechanical properties of the asphalt.
At the present time, unfunctionalized polymers are being used in paving and roofing applications. Unfunctionalized polymers have certain disadvantages which can cause problems when used in applications such as these. Such disadvantages include undesirably low adhesion to polar materials such as some asphalts, fillers, aggregates, substrates, reinforcing mats, and the like. There is a need for modifiers which have better adhesion to polar materials.